Day Counter

How to use

1. Enter the start date; leave the end date blank to use today.
2. Read the calendar difference and the total days, weeks, hours and seconds.
3. Check the dates of the next round hundred and thousand days and the next anniversary.
4. Use a future end date to run it as a countdown.

How it works

Calendar-day basis and live refresh

Days are counted as calendar days: the number of calendar days between the start day at 0:00 and the end day at 0:00. It uses the "UTC noon method" (fixing the local date at noon before subtracting) to avoid the +/-1 day jitter from 23-/25-hour daylight-saving switch days. Leaving the end date blank counts to today with a per-second page refresh and a live total-seconds figure; a specific end date gives a fixed-range count.

Month advance and month-end truncation

The "X years X months X days" calendar difference advances by month: estimate total months from the year-month difference, add those months to the start day (month-end overflow truncates to the month's last day, so Jan 31 plus 1 month is Feb 28), step back a month if it exceeds the end day, and the remaining calendar days are the days. For those born Feb 29, common-year anniversaries land on Feb 28 — the convention shared by mainstream calendar libraries (date-fns, moment).

Milestones and anniversaries

Milestones are anchored to the start day: the next round hundred or thousand day is the calendar day of "day N," and anniversaries follow the month-advance rule above. The table shows both the target date and days remaining; if today is the anniversary, it says "today is exactly the Xth anniversary." For a countdown, just set the end date to a future day; reversed dates swap automatically.

Code example

// plain (d2 - d1) / 86400000 yields 23- or 25-hour days in DST time zones -> fractional days
const dayDiff = (a, b) => {
  const toUTC = (d) => Date.UTC(d.getFullYear(), d.getMonth(), d.getDate());
  return Math.round((toUTC(b) - toUTC(a)) / 86400000);
};

dayDiff(new Date('2026-01-01'), new Date('2026-03-01'));   // 59

// adding N days works the same way: build at UTC, read back the calendar fields
function addDays(date, n) {
  const t = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate() + n);
  const d = new Date(t);
  return [d.getUTCFullYear(), d.getUTCMonth() + 1, d.getUTCDate()];
}
addDays(new Date('2026-02-27'), 1);        // [2026, 2, 28]

# Linux (GNU date): convert to epoch seconds, then subtract
a=$(date -d 2026-01-01 +%s)
b=$(date -d 2026-03-01 +%s)
echo $(( (b - a) / 86400 ))

# macOS (BSD date): -j -f takes the input format
a=$(date -j -f %Y-%m-%d 2026-01-01 +%s)
b=$(date -j -f %Y-%m-%d 2026-03-01 +%s)
echo $(( (b - a) / 86400 ))

# what date is 30 days from today?
date -d "+30 days" +%F            # Linux
date -v+30d +%F                   # macOS

FAQ